1. Technical Field
The present invention generally relates to organic light emitting diode display technology fields and, particularly to an active matrix organic light emitting diode display, a pixel circuit and a data current writing method of the active matrix organic light emitting diode display.
2. Description of the Related Art
In regard to an organic light emitting diode (OLED) display, an issue encountered in a low temperature poly-silicon (LTPS) process for manufacturing the OLED display is that threshold voltages of manufactured transistors are not identical with one another, which would result in the existence of difference among currents flowing through respective transistors for driving OLEDs and thereby cause uneven brightness of display. In another aspect, an issue produced in an amorphous silicon thin film process for manufacturing the OLED display is that the threshold voltages of respective transistors for driving the OLEDs would be varied under long time use. In addition, the OLEDs have inherent issue of aging and thus light-emission efficiency would decrease along the increase of using time.
In order to improve the influence associated with brightness caused by the above-mentioned factors, U.S. Pub. No. 2008/0136338 discloses an improved active matrix OLED display, the disclosure of which is fully incorporated herein by reference. Referring to FIG. 1, the active matrix OLED display includes a control circuit 21, a data line 22, a power line 24 and a plurality of pixels 23. Moreover, the control circuit 21 includes a source/sensing module 211 and a data programming module 213.
The source/sensing module 211 includes an amplifier Amp1, P-type transistors Msense and Msource, a switching transistor MS1 and a capacitor CS1. An output terminal of the amplifier Amp1 is electrically coupled to the gate of the transistor Msense and further electrically coupled to the gate of the transistor Msource through the switching transistor MS1, an non-inverting input terminal of the amplifier Amp1 is electrically coupled to a constant voltage Vcol, and an inverting input terminal of the amplifier Amp1 is electrically coupled to a node nc. The node nc stays constant at the voltage Vcol except for small variation during programming. When the switching transistor MS1 is turned ON, gate voltages of the respective transistors Msense and Msource are established by a current, which flows through the transistors Msense and Msource in response to the current line 24. When the current line 24 starts drawing more current, the node nc and correspondingly the inverting input terminal of the amplifier Amp1 voltage change. Hence in response to any node nc voltage change, the amplifier Amp1 regulates the gate voltages of the respective transistors Msense and Msource to regulate a current flowing through the transistors Msense and Msource. The resulting change in the voltage at the output terminal of the amplifier Amp1 changes the gate voltages of the respective transistors Msense and Msource until the current supplied by both the transistors matches the drawn current. In addition, the capacitor CS1 is electrically coupled between the gate and the drain of the transistor Msource, so that the gate voltage of the transistor Msource stays constant when the switching transistor MS1 is turned OFF.
The data programming module 213 is electrically coupled to the source/sensing module 211. The data programming module 213 includes an amplifier Amp2, a switching transistor MS2 and a capacitor CS2. An output terminal of the amplifier Amp2 is electrically coupled to the data line 22, an non-inverting input terminal of the amplifier Amp2 is electrically coupled to the capacitor CS2 and further electrically coupled to the gate of the transistor Msense through the switching transistor MS2, and an inverting input terminal of the amplifier Amp2 is electrically coupled to the gate of the transistor Msense. In a sampling period, the switching transistor MS2 is turned ON to sample the voltage at the gate of the transistor Msense and stores it in the capacitor CS2.
Each of the pixels 23 has a circuit configuration of 2T1C (i.e., two-transistor-one-capacitor) and specifically includes an N-type driving transistor M21, a switching transistor M22, an OLED 232 and a storage capacitor Cs. The gate of the driving transistor M21 is electrically coupled to the data line 22 through the switching transistor M22, the source of the driving transistor M21 is electrically coupled to a positive terminal of the OLED 232, and the drain of the driving transistor M21 is electrically coupled to the current line 24. The storage capacitor Cs is electrically coupled between the gate and the source of the driving transistor M21.
During a programming period, a single pixel in one pixel column is selected and the switching transistor M22 of the selected pixel is turned ON. The source/sensing module 211, the data programming module 213 and the driving transistor M21 of the selected pixel 23 constitute a feedback loop through the current line 24 at the node nc and the data line 22. When an external data current Idata is injected into the node nc, using the transistor Msense of the source/sensing module 211 to sense node nc voltage change and providing a particular data voltage (i.e., generally programmed data voltage) by the output terminal of the amplifier Amp2 of the data programming module 213 to drive the gate of the driving transistor M21, until the current drawn by the driving transistor M21 from the current line 24 matches the injected data current Idata. As a result, a pixel current of the selected pixel is compensated (i.e., generally an updated pixel current is written).
However, for the above-mentioned active matrix OLED display, since the current line 24 is used for both current sensing and power supplying, although only one pixel in one pixel column is selected to perform the pixel current compensation during the programming period, the driving transistors of the other non-selected pixels all still have currents flowing therethrough so that the current on the whole current line is extremely large while the current flowing through the selected pixel relatively is considerably small. As a result, it is difficult to distinguish the current for compensating the selected pixel from another current caused by noise and thus the compensation accuracy of pixel current is unsatisfactory.